Recently, the rapid development of scientific technology has improved the quality of life of the entire humanity and caused many changes in the medical environment. After a medical image, such as X-ray, computed tomography (CT), functional magnetic resonance imaging (fMRI), etc., is taken at a hospital, it may take several hours or days before the medical image can be interpreted. However, a picture archiving and communication system (PACS) was introduced that allows for a medical image to be sent to the monitor screen of a radiologist by the PACS and then immediately interpreted. In addition, a lot of ubiquitous healthcare-related medical devices that enable a user to check his or her blood glucose and blood pressure without visiting a hospital have been introduced. Therefore, patients with high blood glucose or high blood pressure are using these medical devices at their home or office.
In particular, high blood pressure is a major risk factor for various diseases, and its prevalence rate is increasing. There has been a need for a system that can continuously measure blood pressure and provide the measured blood pressure in real time. As an example of a method for measuring blood pressure, ubiquitous healthcare (u-Health) has been used. In u-Health, a blood pressure measuring sensor is inserted into the pulmonary artery of a patient with a chronic heart disease to measure blood pressure in real time, and the measured blood pressure is sent to the patient's doctor using wireless communication. The doctor may remotely monitor any change in the blood pressure of the patient's pulmonary artery and may send a prescription to the patient. This technology can dramatically reduce patients' visits to hospitals. However, although the technology has the advantage of measuring blood pressure continuously and accurately, blood pressure is measured in an invasive manner in this technology. The technology thus entails a difficult medical procedure and runs the risks of arterial damage and possible contamination.
Therefore, interest has remained in a system that can measure blood pressure in a non-invasive manner and in real time without inserting a blood pressure measuring sensor into an artery blood vessel. Additional research has been conducted on methods of monitoring blood pressure in the ubiquitous environment and providing measured blood pressure to a user as a bio-feedback so that the user can monitor and may be able to control his or her blood pressure. A technology that employs a method of measuring blood pressure by attaching a cuff to an arm has also been used. In this technology, however, someone (a patient or another person) has to operate a blood pressure measuring device each time a blood pressure measurement value needs to be obtained. Therefore, continuous measurement of blood pressure is difficult.
In particular, in order to quickly inform a patient of the risk of high blood pressure so that the patient can receive emergency treatment within a short time, a technology is needed that can measure blood pressure continuously and inform a patient of the blood pressure measurement result in real time to enable the patient himself or herself to prevent and/or manage high blood pressure. A system may be used that includes a non-invasive sensor, which can measure signals such electrocardiogram (ECG), photoplethysmography (PPG) and saturation of peripheral oxygen (SpO2), in a device wearable on a human body. The system can monitor blood pressure in real time by estimating a blood pressure level by processing the above signals. A method of estimating blood pressure based on measured body signals is disclosed in Korean Patent Application Nos. 2013-116158 and 2013-116165, incorporated herein by reference in their entirety.
FIG. 1 illustrates related art in the form of an embodiment of a blood pressure measuring method. Referring to FIG. 1, a main body of a blood pressure measuring device includes a display A, a first electrode B, and a second electrode C. The first electrode B for body signal measurement is installed on a back of the main body (e.g. an inner surface that contacts a wrist wearing the main body of the blood pressure measuring device), and he second electrode C for body signal measurement is installed on a front of the main body (e.g. an outer surface that does not contact the wrist wearing the main body of the blood pressure measuring device). If the user touches the second electrode C with a part (such as a finger) of his or her body while the first electrode B contacts a user's wrist wearing the main body, the user's ECG signal can be measured by the first electrode B and the second electrode C. Further, using a measuring terminal (not illustrated in FIG. 1), the blood pressure measuring device can be connected to sensors for measuring PPG and SpO2 signals. Then, a blood pressure level calculated based on the measured body signals may be displayed on the display A for the user.
Hereinafter, ECG, PPG and SpO2 (i.e. examples representative of “multiple body signals” referred to throughout) will be described in more detail. The “multiple body signals” may include other body signals in addition to or instead of the above signals, though. An ECG is a waveform that represents the vector sum of action potentials generated by a special excitatory and conductive system. That is, the ECG is a vector sum signal, measured using electrodes attached onto the human body, of action potentials generated by each component of the heart such as sinoatrial (SA) node, atrioventricular (AV) node, His bundle, bundle branches, Purkinje fibers, etc. For example, an ECG signal can be obtained using a standard limb lead method.
A PPG is a pulse wave signal measured in peripheral blood vessels when blood ejected during ventricular systole is delivered to the peripheral blood vessels. A PPG signal can be measured using optical characteristics of biological tissue. For example, a PPG sensor (a photo sensor) that can measure a pulse wave signal may be attached to a location (such as a fingertip or a tip of a toe) where the peripheral blood vessels are distributed. Then, the PPG sensor may measure a PPG signal by converting a change (a volume change) in the blood flow rate of the peripheral blood vessels into a change in the amount of light. The PPG signal can be measured by irradiating red light generated by a light-emitter of the PPG sensor to the human body and observing a change in the amount of light reflected by the human body and then received by a light receiver. Generally, information such as pulse transit time (PTT) or pulse rave velocity (PWV) is extracted by analyzing the correlation between a PPG signal and an ECG signal, instead of the PPG signal only, and cardiovascular diseases are diagnosed based on the extracted information. A characteristic point may be obtained by performing a quadratic differential on a PPG signal, and PTT and PWV signals may be extracted by measuring a time interval from a peak (R wave) of an ECG signal. Then, the extracted PTT and PWV signals are used to diagnose the state of blood vessels, hardening of the arteries, peripheral circulatory disturbance, etc.
SpO2 is a body signal indicating oxygen content in hemoglobin from among various components of blood. SpO2 can be measured by sequentially irradiating red light and infrared light to an area of peripheral blood vessels of the human body in each period and observing a change in the amount of light reflected by the human body and then received by a light receiver. For example, SpO2 can be measured using the PPG sensor (the photo sensor) described above.